There has been so-called idling stop control to be performed in an engine mounted on a vehicle. In the idling stop control, the engine is stopped when predetermined stop requirements are satisfied, and is restarted when predetermined start requirements are satisfied. In this manner, the fuel consumption and the exhaust gas amount are reduced.
In the idling stop control operation, when automatic start requirements are satisfied, a starter relay for activating the starter of the engine is energized by a starter energizing circuit, so as to automatically restart the engine (see Reference 1, for example).
The starter energizing circuit normally has two transistors connected in series between the starter relay and the power supply. When the automatic start requirements are satisfied, the starter energizing circuit turns on the two transistors (makes the two transistors conductive) at the same time, so as to energize the starter relay. With the two transistors being provided in series, continuous energization of the starter relay can be prevented by turning off the other transistor, when one of the transistors has ON failure.
[Reference 1] Japanese Unexamined Patent Publication No. 2000-145491